7 - Neuromuscular and spinal cord

Question 1

What two inputs can the membrane potential of post-synaptic neurone be altered by?

Answer 1

Excitatory post synaptic potential (EPSP) - made less negative - depolarisation
Inhibitory post synaptic potential (IPSP) - made more negative - hyperpolarisation

Question 2

What is the neurotransmitter for the NMJ?

Answer 2

ACh

Question 3

Describe the process that takes place at the NMJ that triggers the action potential in the muscle fibres?

Answer 3

action potential arrives —–> Ca2+ influx —–> ACh release —–> binds to receptors on motor end plates —–> propagates action potential (Na+ channels open) —–> actin and myosin (muscle contraction)

Question 4

What are mEPPS?

Answer 4

mini end plate potentials
at rest, individual vesicles in the presynaptic membrane release ACh at a very slow rate leading to small fluctuations in the post-synaptic membrane potential

Question 5

What are the alpha motor neurones?

Answer 5

the lower motor neurones of the brainstem and spinal chord

Question 6

What kind of muscle fibres do alpha motor neurones innervate?

Answer 6

extrafusal muscle fibres (standard skeletal muscles that contract)

Question 7

What is the difference between intrafusal and extrafusal muscle fibres?

Answer 7

Intrafusal – these are skeletal muscle fibres that serve as sensory organs (proprioceptors) that detect the amount and rate of change of length of a muscle
Extrafusal – standard skeletal muscle fibres that are innervated by alpha motor neurones and generate tension by contracting, thereby allowing for skeletal muscle movement

Question 8

What is a motor neurone pool?

Answer 8

Collection of lower (alpha) motor neurones that innervate a single muscle

Question 9

Define motor unit

Answer 9

A single motor neurone together with all the muscle fibres that it innervates - it is the smallest functional unit that can generate force.

Question 10

Describe the organisation of cell bodies in the ventral horn?

Answer 10

• those that innervate flexor muscles are in the posterior part
• those that innervate extensor muscles are in the anterior part
• those that innervate proximal muscles are more medial and those that innervate distal muscles are more lateral

Question 11

What is an important rule to remember regarding the connections between alpha motor neurones and muscle fibres?

Answer 11

One motor neurone can innervate several muscle fibres

But every muscle fibre can only be innervated by one motor neurone

Question 12

Under what conditions can this rule be broken?
(That:
one motor neurone can innervate several muscle fibres, but every muscle fibre can only be innervated by one motor neurone)

Answer 12

Under pathological conditions (e.g. severed nerve), the axonal regeneration can result in the innervation of muscle fibres that are already innervated

Question 13

Describe and explain the difference in innervation ratio across different muscles in the body using examples.

Answer 13

Muscles that require very fine control (e.g. extrinsic eye muscles) have a low innervation ratio (few fibres innervated by a single neurone)
Muscle that are required to generate a lot of power have a high innervation ratio because when the motor unit fires, it will cause the contraction of a large mass of muscle fibres thus generating power (e.g. quadriceps)

Question 14

What are the 3 types of motor unit?

Answer 14

Slow (S)

Fast (FR/type IIA and FF/type IIB) - fatigues easily and resistant to fatigue

Question 15

Describe the properties of the different types of motor units when they are stimulate?

Answer 15

slow - generate little force, but over a long period of time (e.g. standing)
fast - generate more force but cannot sustain it for very long

Question 16

Describe the structural and functional differences between slow and fast twitch muscle fibres.

Answer 16

Slow fibres have:
•	Smallest diameter cell bodies 
•	Small dendritic trees
•	Thinnest axons 
•	Slowest conduction velocity 
Fast fibres have:
•	Larger diameter cell bodies 
•	Large dendritic cells 
•	Thicker axons 
•	Faster conduction velocity

Question 17

What are the two mechanisms by which the brain regulates the force that a single muscle can produce?

Answer 17

RECRUITMENT:
recruiting more motor units for the muscle contraction using the ‘size principle’
RATE CODING:
increasing the frequency of action potentials travelling down the nerves to the muscle fibres

Question 18

What is the ‘size principle’ that governs recruitment?

Describe the order of recruitment of motor units with increasing force generation.

Answer 18

Smaller units are recruited first, which are generally slow fibres

Slow -> Fast Fatigue-Resistant -> Fast Fatiguable

Question 19

What are neurotrophic factors?

Answer 19

They are a type of growth factor produced within the nerves and are transported throughout the nerve to maintain the nerves integrity and function.
They are a type of growth factor that prevents neuronal death and promotes the growth of neurones after injury.

Question 20

describe the regeneration of neurones in the nervous system

Answer 20

CNS neurones don’t regenerate after injury

neurones in a peripheral environment allow for axonal regeneration

Question 21

When a fast nerve is transplanted onto a slow muscle fibre, the fibre becomes fast. What does this show?

Answer 21

This shows that the function of the muscle fibre is very much determined by the type of nerve that innervates it.
The action potentials can’t be the only thing being delivered to the muscles by the nerves.

Question 22

How easy is it to switch from one motor unit type to another?

Answer 22

Type 2B to Type 2A can happen with training
There is usually no way of changing from type 2 to type 1 or vice versa except in the case of severe deconditioning e.g. zero gravity or spinal injury

Question 23

How does muscle composition change with ageing?

What is the generic name for this?

Answer 23

Ageing is associated with a loss of type 1 and 2 fibres (preferentially loss of type 2 fibres)
So a large proportion of muscle fibres in ages muscle are type 1

This loss of muscle is called sarcopenia

Question 24

What tract is responsible for voluntary movements?

Answer 24

Pyramidal/Corticospinal tract

Question 25

What is the role of extrapyramidal tracts?

Answer 25

It is responsible for automatic movements in response to stimuli (movements body makes without being aware)

Question 26

What is a reflex?

Answer 26

Automatic and often inborn response to a stimulus that involves a nerve impulse passing inward from a receptor to a nerve centre and then outwards to an effector (a muscle or gland) without reaching the level of consciousness.

Question 27

How can reflex testing help determine whether there has been sensory or motor loss?

Answer 27

If the muscle can voluntarily contract, this indicates that there is nothing wrong with the motor neurone.
If a reflex is stimulated by hitting the tendon, and there is no reflex/response seen, this indicates a sensory defect.

Question 28

Describe how reflexes can signal to both muscle in a pair.

Answer 28

Muscles work in pairs – flexors and extensors. An afferent signal will travel to the spinal cord. The afferent stimulates certain motor neurones (e.g. those supplying the flexor), and inhibits others (e.g. those supplying the extensor) to allow for movement.

If the afferent fibres from the muscle are stimulated you will get a monosynaptic connection with the efferent to get contraction (excitatory). It can also synapse with an interneuron, which inhibits the motor neurone supplying another muscle (inhibitory).

In general terms: there is an inhibitory signal to the antagonist at the same time as the excitatory signal to the agonist.

Question 29

What are the two signals that are generated when the patellar ligament is tapped?

Answer 29

There is an excitatory signal going to the quadriceps

There is also an inhibitory signal going to the hamstrings (antagonist)

Question 30

Why is there a difference in the time taken for these signals to reach the relevant muscles?

(When the patellar ligament is tapped, there is an excitatory signal going to the quadriceps and an inhibitory signal going to the hamstrings (antagonist))

Answer 30

The signal going to the quadriceps only has one synapse (monosynaptic) whereas the signal to the hamstrings goes via an inhibitory interneurone so there are two synapses.
This means that the signal to the quadriceps arrives slightly faster than the signal to the hamstrings.

Question 31

Name the standard reflex test and explain why is used as standardisation

Answer 31

The Hoffman Reflex - used clinically to assess which set of nerves may be affected by a disorder.
the stimulus can be identical every time the reflex is tested (any change in reflex size is NOT due to the input)

bypassing the stretch that is caused by the tendon hammer on the patellar ligament and directly stimulating the nerve, which has sensory and motor fibres.
This would mean that the stimulus is identical every time (in duration and amplitude) and the magnitude of the reflex elicited will not be due to variations in input (how hard you tap the patellar tendon/where you tap it)

Question 32

What are the two twitches that are seen when you stimulate the nerve behind the knee?

Answer 32

M wave – direct motor response - motor neurone is directly stimulated
H wave – reflex action when sensory nerve are stimulated

Question 33

Why do sensory nerves show a response at lower stimulus intensity than motor nerves?

Answer 33

Sensory nerves are more amenable to electrical stimuli because they are larger so you can get a response from a sensory nerve (H wave) at lower stimulus intensity than the M wave.

Question 34

What names are given to the opposing actions of antagonistic muscle pairs during polysynaptic reflexes?

Answer 34

Flexion withdrawal

Crossed extensor

Question 35

Describe the supraspinal control of reflexes.

Answer 35

descending control over reflexes - we can influence reflexes via voluntary command
Higher centres within in CNS can exert inhibitory and excitatory regulation in some reflexes (especially in stretch reflexes)

Question 36

What is the Jendrassik manoeuvre?

Answer 36

Tap someone’s patellar tendon with a tendon hammer whilst they are clenching their teeth. The response elicited is 2-3 times greater.

Question 37

If you decerebrate an animal (but keep them alive) and test their reflexes, what would you expect to observe?

Answer 37

Hyperreflexia

Increased muscle tone

Question 38

What is the gamma reflex loop?

Answer 38

It shortens the spindles in muscle to maintain its sensitivity
There is also facilitation from higher centres, which increase the sensitivity of the motor neurone to afferent input

Question 39

What signs are seen with upper motor neurone lesions?

Answer 39

Hyperreflexia
Clonus
Babinski’s Sign